Method Overriding Revisited

Part of the Object Oriented Programming section of Coddy's GDScript journey. Lesson 21 of 65.

Polymorphism means "many forms": one method call does different things depending on the class of the object that receives it. Overriding is what makes this work.

# animal.gd
class_name Animal
extends RefCounted

func speak() -> String:
	return "..."

# dog.gd
class_name Dog
extends Animal

func speak() -> String:
	return "Woof!"

# cat.gd
class_name Cat
extends Animal

func speak() -> String:
	return "Meow!"

An Array[Animal] can hold all of them. The loop calls speak() without knowing which class each element has, and every object answers with its own version:

var animals: Array[Animal] = [Dog.new(), Cat.new(), Animal.new()]
for animal in animals:
	print(animal.speak())

Woof!
Meow!
...

A function with an Animal parameter works the same way. It accepts every child class, including classes you add later, without a change:

func make_it_speak(animal: Animal) -> void:
	print("It says ", animal.speak())

make_it_speak(Cat.new())

It says Meow!

Compare this with checking classes one by one. A chain of if animal is Dog: branches must grow with every new class. With overriding, each class carries its own behavior, and the code that calls it stays the same.

Challenge

Easy

A garage handles every vehicle through one Array[Vehicle]. In vehicle.gd, wheels() returns 0 and sound() returns ....

Write describe() in Vehicle. It returns BRAND: N wheels, SOUND, calling wheels() and sound().
Car overrides wheels() with 4 and sound() with Vroom.
Motorcycle overrides them with 2 and Brrm.
Truck already stores its number of trailers. It overrides wheels() with 6 plus 4 per trailer, and sound() with Honk.

The children do not override describe(). The supplied code reads how many vehicles follow, then one line per vehicle: car BRAND, motorcycle BRAND, truck BRAND TRAILERS or vehicle BRAND. It prints every describe() and the total of wheels(). For car Toyota and truck Volvo 2 it prints:

Toyota: 4 wheels, Vroom
Volvo: 14 wheels, Honk
Total wheels: 18

Your code goes in vehicle.gd, car.gd, motorcycle.gd and truck.gd. main.gd holds the supplied input/output code and cannot be edited.

Try it yourself

extends Node

func _ready():
	# Supplied input/output code: keep it as it is
	var count := int(OS.read_string_from_stdin())
	var vehicles: Array[Vehicle] = []
	for i in count:
		var p := OS.read_string_from_stdin().split(" ")
		if p[0] == "car":
			vehicles.append(Car.new(p[1]))
		elif p[0] == "motorcycle":
			vehicles.append(Motorcycle.new(p[1]))
		elif p[0] == "truck":
			vehicles.append(Truck.new(p[1], int(p[2])))
		else:
			vehicles.append(Vehicle.new(p[1]))
	var total := 0
	for v in vehicles:
		print(v.describe())
		total += v.wheels()
	print("Total wheels: %d" % total)